The present invention relates to a photomultiplier tube, and more particularly, to geometrical arrangement of an electrode structure of the photomultiplier tube for reducing a length of the tube.
In a photomultiplier tube, minute incident light is received on a photocathode, and photoelectron generated at the photocathode is multiplied through a secondary electron multiplier system for taking out an multiplied electric signal.
The photomultiplier tube is widely assembled and used in various radiation detectors and spectrometers such as a scintillation counter. In accordance with recent demand in down sizing of these detectors, a compact photomultiplier tube is required, particularly reduction in a tube length thereof is required.
A conventional box and grid combination type photomultiplier tube generally includes an outer tube or envelope in which provided are a photocathode, a first dynode, a focussing electrode, an anode, and a plurality of dynodes. Bundle of lights pass through substantially entire surface of the photocathode positioned at a head of the tube. The first dynode receives photoelectrons ejected from the photocathode, and a focussing electrode is adapted for converging the photoelectrons onto the first dynode. To this effect, the focussing electrode is formed with a transmission hole through which photoelectrons pass. Since the light bundles pass through substantially entire surface of the photocathode, the transmission hole and the first dynode are preferably be positioned at a central axis of the tube in order to effectively direct photoelectrons converted at photocathode toward the first dynode. The anode is disposed in a vicinity of a tube bottom, and the plurality of the dynodes are disposed between the first dynode and the anode and are arrayed approximately linearly in a lengthwise direction of the tube.
As described above, the focussing electrode is formed with the photoelectron transmission hole whose center is positioned coaxially with the central axis of the tube. Thus, according to the conventional box and grid combination type photomultiplier, large length of the photoelectron multiplier system, particularly, large length of the plurality of the dynode groups is provided in the axial direction of the tube, since these dynodes group must also be arrayed in the axial direction of the tube. Therefore, resultant tube length of the photomultiplier tube becomes large.
In order to overcome this problem, various proposals have been made for reducing the tube length. For example, a conventional photomultiplier tube is described in Japanese Patent Publication No. 60-30063 as shown in FIG. 1. According to the conventional example, there is provided a box and grid combination type photomultiplier tube which includes a photocathode 112, a focussing electrode 113 formed with a photoelectron transmission hole at a central portion thereof, a first box type dynode 114, grid type dynodes 117, 118, 119, 120 arrayed in a direction perpendicular to an axial direction of a tube 111 and positioned below the first box type dynode 114, and second and third box type dynodes 115 and 116 for directing and multiplying secondary electron from the first dynode 114 to the grid type dynodes 117 through 120. The second and third box type dynodes 115 and 116 are arrayed in the axial direction of the tube 111. More specifically, the second dyonode 115 is positioned beside the first dynode 114, and the third dynode 116 is positioned beside the grid type dynodes 117 through 120.
In this arrangement, the photoelectron transmission hole formed in the focussing electrode 114 has a center point coincident with a central axis of the tube 111, and a center of the first dynode 114 positioned below the transmission hole is also provided coaxially with the central axis of the tube in order to obtain sufficient converge of the electrons onto the dynode 114. On the other hand, the array of the grid type dynodes 117 through 120 is oriented in a direction perpendicular to the central axis of the tube.
In this photomultiplier tube, entire length of the dynodes in the axial direction of the tube is a sum of a lengths of the first dynode 114 and third dynode 116 or the grid type dynodes 117 through 120. Thus, entire axial length of the tube can be advantageously reduced, since the grid type dynodes array is not oriented in the axial direction of the tube but is oriented transversely relative to the tube axis.